The invention relates to a process for the enhanced oil recovery of underground mineral oil deposits by selective, reversible reduction of the permeability in hot-water and/or steam flooding.
Steam flooding and hot-water flooding are thermal EOR processes (EOR=Enhanced Oil Recovery) for the winning of mineral oil from deposits and are used predominantly for the production of relatively heavy oils or tars. In order to achieve a good ultimate recovery, the ratio M of the mobility of the flooding medium (steam or water) to the oil mobility must be lowered. This is performed in steam flooding and in hot-water flooding by heating up the deposit utilizing the heat content of the flooding media. Since the oil viscosity usually drops more under a temperature increase than the viscosity of the flooding media, the mobility ratio M improves and an increase in the ultimate oil recovery of the deposit is thereby achieved.
In the ideal case of steam or hot-water flooding, the heated zone spreads evenly in the deposit and the flooding medium displaces the oil evenly to the production wells. However, in practice this is never the case as deposits are not homogeneous and consist of beds having different properties and which may be separated from one another by impermeable embedments. Channels are produced, in which part of the flooding medium advances faster and reaches the production well prematurely. This is to be prevented, for example according to U.S. Pat. No. 4,250,963 by the steam being accompanied by a monomer, eg. styrene, which condenses at the temperature front and, at a temperature below about 120.degree. C., polymerizes to form a polystyrene. The disadvantage of this and many similar polymerization methods lies in the fact that the polymerization is irreversible, meaning that the deposit remains blocked for ever more at this point.
According to U.S. Pat. No. 4,232,741, a very complicated process utilizing a nitrogen-generating substance, a surface-active substance, a pH control system and an acid-liberating substance is used to produce a foam at a desired point of the underground formation, likewise blocking the formation. Apart from the fact that it is very difficult to instigate such a complicated process at a very precise point in a largely unknown underground formation, the foam formation causes irreversible blocking, which lasts as long as the foam exists. After decomposition of the foam, the formation is permeable again at this point and channel formation can occur again.
Another reason for channel formation, and thus for inadequate oil recovery of the deposit lies in the fact that steam has a substantially lower density than oil and the tongue of steam therefore has a propensity to move into the upper part of the deposit bed and to reach the production well in the form of a narrow finger. According to Canadian Patent Specification No. 1,080,614, this is to be prevented by separating the injected steam, which usually contains a greater or lesser proportion of water, into a gaseous and a liquid phase, and introducing the liquid phase into the formation above the gaseous phase. Apart from the additional expenditure on equipment, here too the gas phase has the propensity to drift upwards and compete with the liquid phase, and the bottom part of the deposit is again avoided by the flooding medium. If the deposit is inhomogeneous, and this inhomogeneity cannot be predicted, as is normally the case, the gas phase forms unforeseen fingers and again runs ahead of the liquid phase with the formation of channels.
In contrast with this, it has now been possible to discover a process for the oil recovery of underground deposits in which the permeability of an underground formation is reduced selectively at the points of highest permeability and in a reversible form. The reduction in the permeability of the rock for the flooding medium can be equated with a reduction in the ratio M of the mobility of the flooding medium to the oil mobility, which results in an increase in the ultimate oil recovery.